You have to remember that in OLD the women are reading dozens of messages at a time. Your message needs to be the odd one in the crowd that gets her attention. Here is one I might use and the particulars of what it contains:
"Hey (her first name if she posted it) Lefty here. I looked at your profile and you don't seem like a gold digger or mental case so today is your lucky day, I would like to shoot the sh|t with you. Congrats. Hope to hear from you by the 32nd of this month? Lefty"
"Hey (first name)" - Lets her know you looked at her profile, not just pic. They like that.
"you don't seem like a gold digger or mental case" - A small compliment
"today is your lucky day" - c0cky/funny
"I would like to shoot the sh|t with you" - to the point
"congrats" - confident
"hope to hear from you by the 32nd of the month" - mysterious. Nearly all will ask "What does that mean?" I respond "I am a man of many mysteries, you will have to talk to me a little longer before I tell you that"
This is just one sample of what I might do. Use this or write your own, but use the elements I used in mine and keep it short and sweet.